Engineering a Fan-Wall Solution

When the aging air-handling units (AHUs) at the Art Institute of Chicago (AIC) needed to be replaced, the AIC's project manager, C.Lee; the executive director of the AIC's physical plant, William Caddick; and the team at McGuire Engineers (MEPC) developed an uncommon solution that included fan-wall technology.

The decision to install fan walls in the AIC was unique because the technology previously had not been used in museums and was not common in Chicago. As part of a major overhaul of the museum's mechanical, electrical, and structural systems, C.Lee performed due diligence and provided overall supervision and management of the process. The museum's curators and conservators advised on the temperature and humidity conditions that needed to be maintained. Caddick and the MEPC team studied the approach carefully and planned and executed the design and installation.

This article discusses how fan walls solved many of the problems usually associated with upgrading AHUs in older structures while increasing efficiency and control of temperature and humidity.

NEW TECHNOLOGY WITH VARIED APPLICATIONS

A fan wall uses many specially designed, high-efficiency, small-diameter, direct-driven plenum fans, which are mounted in an array within an air handler's air-way tunnel.

Implemented over the last 10 to 15 years, fan walls primarily have been a West Coast phenomenon, installed mainly in semiconductor facilities. Recently, however, fan walls have been incorporated into a variety of commercial buildings. Any type of structure that requires specific temperature and humidity conditions is a candidate for a fan wall. Currently, structures utilizing fan walls include hospitals, laboratories, cleanrooms, research facilities, libraries, and a major U.S. football stadium. The AIC was the first museum to install fan-wall technology.

FAN WALLS SOLVE MULTIPLE CHALLENGES

The AIC is an older structure, built during the 1890s on top of rubble created by the Great Chicago Fire of 1871. It is a mix of old and contemporary architecture, including various spaces that were added over the last several decades and a new wing that is under construction. As might have been expected, the building presented many challenges when the need to upgrade and replace existing AHUs serving the galleries throughout the building arose. Not only did the fine artwork and artifacts require proper temperature and humidity conditions to preserve their appearance and physical integrity, comfortable conditions had to be maintained for the public.

The AHUs had been in service in some instances for 50 years and, as a result, had become maintenance-intensive. In preemptively replacing the old units, the engineers needed to consider a number of factors, including the small size of the entryways, the interstitial spaces used for mechanical and other equipment, and future accessibility of the system for maintenance and repairs. This task was made even more challenging by the location of the units within the structure and the physical size of the units being replaced, which ranged from 10,000 to 30,000 cfm.

After a detailed analysis, MEPC recommended fan walls as the best of several potential solutions for the replacement of the air handlers. The fan-wall option risked the least amount of downtime and offered the best opportunity for a rapid return to normal system operation. Also, it minimized the need to remove or demolish any of the public spaces within the AIC to facilitate equipment replacement. Caddick and MEPC developed a plan to replace the aging units in a specific sequence to ensure that there would be minimal disruption to museum exhibit space and scheduled events. Removal and replacement required precision equipment and labor scheduling and staging to minimize the downtime for each system and area served by a unit being replaced.

In addition to meeting the challenges of installing a new system in an existing structure, the designers identified other benefits of the multiple-fan-array solution. The system featured extremely low radiated and airborne noise levels and virtually eliminated vibration caused by rotor imbalance and airway-tunnel turbulence. Other design features included component portability, a smaller AHU footprint, and, perhaps most importantly, redundancy and reliability that was not achievable with conventional fan-system-replacement options. Under Caddick's direction, the replacement AHUs were configured so that each unit would have built-in N+1 redundancy. This meant each system could suffer a fan failure and continue to deliver the required design airflow for space conditioning. The direct-driven fans used in this technology also eliminated the sometimes troublesome maintenance of belt-driven fans.

The engineers' recommendation to move forward with the multiple-fan approach made the positioning of replacement air handlers — knocked down for field reassembly by factory-supervised crews — much easier than if a conventional fan system had been installed. All components, including the fans, were no more than 28-in. wide, making them easy to transport to the assembly sites using existing routes of access, including building elevators, catwalks, and service corridors, without the necessity of building demolition. The small component size was a huge factor in maintaining the tight replacement schedule for each air handler and reducing overall project costs when compared with alternative solutions. In addition to reliability and ease-of-installation benefits, higher energy efficiency, reduced low-frequency noise, and minimal vibration levels were achieved.

During the past two-and-a-half years, seven systems have undergone AHU replacement. The results have received high praise, and the curators and conservators have been happy not only with the temperature and humidity conditions, but the improved acoustical conditions. The AIC staff is requiring that this technology be used in its new, modern wing.

DIAGNOSING PROBLEMS

The number of fans in each fan-wall system varies based on airflow requirements. In one typical arrangement, the art museum uses walls of nine fans in a three-by-three configuration. The status of each fan is monitored electronically, and a collective reading of each fan wall's frequency, total airflow, and speed is indicated on a motor control panel. As mentioned previously, MEPC and the AIC designed the fan walls to have some redundancy. Normal operations require only eight fans to function, so the ninth fan is covered with a blank-off plate. If an active fan fails, the blank-off plate is moved to the inactive fan until it is repaired or replaced. If two fans fail, the remaining fans can be ramped up.

In the event of fan failure, an electronic signal is sent via computer to building engineers, indicating which fan is not operating optimally. During repairs, the rest of the fans can be adjusted to maintain necessary temperature and humidity conditions.

CONCLUSION

Because of their installation flexibility, operations and maintenance efficiency, and excellent track record for maintaining specific air-quality and acoustical conditions, fan walls are a useful solution for a variety of structures. As the technology continues to improve, fan walls should be considered as a solution for retrofitting old fans as well as for new construction.

With more than 25 years of experience designing HVAC and plumbing systems for commercial and institutional buildings, William Stangeland is a senior vice president for McGuire Engineers overseeing projects worldwide. He is affiliated with numerous professional organizations and is a member of the Chicago High Rise Committee and the Chicago Building Congress. He can be reached atwstangeland@mepcinc.com. After graduating from Loras College in Dubuque, Iowa, in 2001, Michael Murphy joined McGuire Engineers as a mechanical engineer. His six years of experience include designing HVAC systems for institutional and commercial buildings, laboratories, and educational facilities. He is an associate member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers.